The invention has been developed primarily for carrying alternating current and will be described hereinafter with reference to that application. It will be appreciated, however, that the invention is not limited to that particular field of use and is also suitable for carrying direct or non periodically varying current.
Tapes comprising superconducting material, and referred to as superconducting tapes, are already known, and comprise one or many superconducting filaments in a medium of silver or silver alloy. Superconducting tapes are used to make coils, magnets, transformers, motors and generators as well as current carrying cables. The main class of superconducting tape is referred to as powder-in-tube or PiT tape. This tape is made by drawing or otherwise reducing a tube of silver, or less usually silver alloy, which is filled with a powder form of a superconducting oxide. The tube is then subjected to further rolling to form it into a thin tape. Multifilamentary tapes are mostly made by grouping a plurality of filled tubes in a common silver or silver alloy sheath at an intermediate stage of reduction.
One important superconducting oxide is known as Bi-2223. This oxide includes bismuth, strontium, calcium, and copper and, as would be known to those in the art, certain limited substitutions can be made. This oxide can be considered a cuprate salt.
Known tapes usually have a thickness of between around 0.2 mm and 0.3 mm, and a width of between 2 mm and 5 mm. The superconducting filaments must be thin to obtain an adequate critical current. A typical thickness is around 10 to 40 microns. Moreover, a typical aspect ratio is at least 1:10.
The filaments comprise many plate-like grains and, for good performance, the grains should be, as much as possible, aligned in the same crystallographic orientation. The relative orientation is often referred to as the grain alignment or “texture”. Thin, well textured filaments allow a high critical current and give overall flexibility to the whole tape.
Composite tapes are sometimes made by forming a stack of individual tapes and wrapping the stack with one or more metal tapes to keep it together. These metal tapes are usually of silver or silver alloy. In U.K. patent application no. 9805641.9 in the name of the same applicant there has been proposed an improved form of composite tape in which the individual tapes are diffusion bonded, eliminating the need for the metal wrapping tape. This allows the elimination of the inevitable gaps and overlapping between the turns of the wrapping tape that create kinks in the filaments that destroy local grain alignment. As foreshadowed above, a reduction in alignment leads to a degradation of the overall critical current density Jc.